Cell size is an important element determining phytoplankton physiological and ecological processes, including nutrient uptake, sinking and grazing; thus, phytoplankton size-structure plays an important role in the structure and function of lake ecosystem. To understand the succession patterns and driving factors of total chlorophyll-a and size structure of phytoplankton, field in-situ investigation was conducted from September 2018 to September 2019 in the east Lake Dongting, a Yangtze River- connected lake. The total phytoplankton chlorophyll-a biomass showed significant seasonality and spatiality. The highest concentration of total chlorophyll-a was observed in summer (22.43 μg/L), followed by autumn (16.95 μg/L), spring (11.69 μg/L) and winter (3.28 μg/L). Spatially, the total phytoplankton chlorophyll-a was highest in the north (26.12 μg/L) and lowest in the east (5.88 μg/L) of the lake. Phytoplankton was overwhelmingly dominated by nanophytoplankton (3-20 μm). The proportion of nanophytoplankton to total phytoplankton chlorophyll-a biomass was highest in winter (68.0%), whereas the proportion of picophytoplankton (0-3 μm) increased gradually in spring and peaked in summer (42.1%). The contribution of microplankton (>20 μm) to total phytoplankton chlorophyll-a biomass was lowest with a mean of 16.2% all year round. The results of redundancy analysis indicated that phytoplankton with different cell sizes showed similar responsive trends to environmental changes, while their adaptabilities were different. Temperature, water levels, nutrients and pH were the key factors explaining size-structure of phytoplankton in east Lake Dongting.